Ebook Neuroanatomy and pathology of sporadic alzheimer’s disease Part 1

Advances in Anatomy, Embryology and Cell Biology

Heiko Braak
Kelly Del Tredici

Neuroanatomy
and Pathology
of Sporadic
Alzheimer's
Disease


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Heiko Braak • Kelly Del Tredici

Neuroanatomy and
Pathology of
Sporadic Alzheimer’s
Disease
With 48 figures


Heiko Braak
Kelly Del Tredici
Zentrum f. Biomed. Forschung AG
Klinische Neuroanatomie/Abteilung Neurologie
Universita¨t Ulm
Ulm
Germany

ISSN 0301-5556
ISSN 2192-7065 (electronic)
ISBN 978-3-319-12678-4
ISBN 978-3-319-12679-1 (eBook)
DOI 10.1007/978-3-319-12679-1
Springer Cham Heidelberg New York Dordrecht London
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.


Preface

The downside of the current tendency to prolonged life expectancy in developed
countries is the increase in diseases associated with advanced age, especially those
involving the central nervous system (CNS). Foremost among these is sporadic
Alzheimer’s disease (AD) which leads to dementia (Brookmeyer et al. 2007; Qiu
et al. 2009; Reitz et al. 2011; Mayeux and Stern 2012). Nevertheless, today, despite
all efforts on numerous fronts, no causal or disease-modifying therapy is available

(Doody et al. 2014; Salloway et al. 2014). AD is a neurological disorder of the
human CNS. The pathological lesions associated with the AD process require an
unusually long period of time to evolve, but, in the final analysis, they result in
clinically recognizable impairment of higher brain functions.
This book is written for a readership that is to some extent familiar with the
anatomy of the human nervous system and is interested in the changes it undergoes during the AD process. As in the previously published book on sporadic
Parkinson’s disease from the same Springer series (Braak and Del Tredici 2009),
the present effort approaches and interprets the pathological process in AD chiefly
from a neuroanatomical perspective. However, we want to make the text readable
for non-experts, inter alia by including throughout it both introductory and more
detailed explanations pertaining to important anatomical relationships that facilitate understanding the material but that are not available in standard textbooks or
only cursorily explained therein, e.g., the anatomy of the entorhinal region.
Clinically, AD only occurs in humans, and the hallmark lesions underlying the
disease process predominantly are found in the human CNS. Thus, there are no truly
adequate animal models for AD (Rapoport and Nelson 2011), although the implications of this reality are largely overlooked in much current research. For the past
25 years, an amyloidocentric understanding of AD research has largely ignored
opposing data and arguments, thereby leaving aside important questions that still
require answers (Maarouf et al. 2010). The authors focus on fundamental aspects of
the AD process as a whole with the intention of encouraging alternatives to the
Ab-centered understanding of AD.
As indicated by its title, this book deals mainly with morphologically recognizable deviations from the normal anatomical condition of the human CNS. The
vii


viii

Preface

AD-associated pathology is illustrated from its beginnings (sometimes even in
childhood) until its final form that is reached late in life. The AD process commences much earlier than the clinically recognizable phase of the disorder and its

timeline includes an unusually extended non-symptomatic phase. The further the
pendulum swings away from the symptomatic final stages towards the early pathology, the more obvious the lesions become, although from a standpoint of severity
they are more unremarkable and, thus, frequently overlooked during routine neuropathological assessment. For this reason, we decided to deal with the hallmark
lesions in early phases of the AD process in considerable detail. Clinically manifest
cases of AD, on the other hand, display extensive disease-associated lesions that, as
a rule, are accompanied by non-AD-related pathologies, including vascular changes
and concomitant neurodegenerative disorders.
For a constitutive introduction to the morphology of AD, one of the authors
(HB) owes a special debt of gratitude to an American colleague, Thomas L. Kemper,
MD (Department of Anatomy and Neurobiology, Boston University School of
Medicine), who also conveyed to him the fascination with the idea that AD is a
disorder that adheres to the conditions of human neuroanatomy. The authors thank
the Goethe University Frankfurt (The Braak Collection). They are also thankful for
valuable comments provided by Khalid Iqbal, PhD (New York State Institute for
Basic Research in Developmental Disabilities) and Michel Goedert, MD (MRC
Laboratory of Molecular Biology, University of Cambridge). They wish to express
their appreciation to Horst-Werner Korf, MD (Dr. Senckenbergische Anatomie,
Goethe University, Frankfurt) for the invitation to prepare this book, Albert
C. Ludolph, MD (Department of Neurology, University of Ulm) for support and
helpful discussions, and Ms. Anne Clauss from Springer (Heidelberg) for careful
editing of the text. They also are grateful to Ju¨rgen Bohl, MD (formerly Department
of Neuropathology, University of Mainz) for ongoing support, Ms. Simone Feldengut (Tables, silver staining, immunocytochemistry), Ms. Siegrid Baumann,
Ms. Gabriele Ehmke, Ms. Julia Straub (immunocytochemistry), Mr. Hans-Ju¨rgen
Steudt (Olympus Germany, Stuttgart) for technical assistance, and Mr. David Ewert
(Department of Neurology, University of Ulm) for the many hours spent preparing
and helping to design the illustrations. In view of the breadth of the subject matter, it
was necessary to weigh the bibliography in favor of more recent original studies and
reviews. In other words, it was not the authors’ intention to supply an exhaustive
survey of all of the pertinent literature from the AD field.
Funding for this work was made possible, in part, by the German Research

Council (Deutsche Forschungsgemeinschaft, DFG) Grant number TR 1000/1-1
and the Robert A. Pritzker Prize from the Michael J. Fox Foundation for Parkinson’s
Disease Research.
This book is dedicated in gratitude to the memories of Eva Braak ({2000),
William R. Markesbery ({2011), and Inge Grundke-Iqbal ({2012).
Ulm, Germany
13 September 2014

Heiko Braak
Kelly Del Tredici


Contents

1

Prologue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

2

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1
Sporadic AD Is a Proteinopathy Linked to the Development of
Intraneuronal Inclusions of Abnormal Tau Protein Which,
in Later Phases, Are Accompanied by the Formation of
Extracellular Plaque-Like Deposits of Amyloid-b Protein . . . . .
2.2
Some Neuronal Types Exhibit a Particular Inclination to the

Pathological Process While Others Show a Considerable
Resistance To It . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3
Consistent Changes in the Regional Distribution Pattern of
Intraneuronal Inclusions Make a Staging Procedure
Possible . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

3

3

6

9

Basic Organization of Non-thalamic Nuclei with Diffuse Cortical
Projections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15

4

Microtubules and the Protein Tau . . . . . . . . . . . . . . . . . . . . . . . . . . .

21

5

Early Presymptomatic Stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1
Stage a: The Appearance of Abnormal Tau in Axons of Coeruleus
Projection Neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2
Stages b and c: Pretangle and Tangle Material Develops in the
Somatodendritic Compartments of Coeruleus Neurons and
Similar Lesions Appear in Additional Brainstem Nuclei with
Diffuse
Cortical Projections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3
Survival of Involved Neurons, Loss of Neuronal Function, and
Degradation of Remnants After the Death of Involved
Neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25
25

28

33

ix


x

6

Contents

Basic Organization of Territories That Become Sequentially
Involved After Initial Involvement of Brainstem Nuclei with
Diffuse Projections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1
The Cerebral Cortex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2
The Amygdala . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3
The Entorhinal Region and the Presubiculum . . . . . . . . . . . . .
6.4
The Hippocampal Formation . . . . . . . . . . . . . . . . . . . . . . . . .
6.5
Cortical Gradients in Differentiation, Myelination,
and Pigmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.6
Interconnecting Pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.
.
.
.
.

37
37
39
41
44

.
.

50
51

.

57

.
.
.

57
61
64

.

70

.

72

8

Alzheimer-Associated Pathology in the Extracellular Space . . . . . . . .
8.1

The Amyloid Precursor Protein and the Abnormal Protein Ab .
8.2
Sources and Secretion of Ab . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3
Transient Extracellular Ab Deposits . . . . . . . . . . . . . . . . . . . . .
8.4
Mature Forms of Ab Deposits and Plaque Degradation . . . . . . .
8.5
Phases in the Development of Ab Deposits . . . . . . . . . . . . . . . .
8.6
Formation of Neuritic Plaques (NPs) . . . . . . . . . . . . . . . . . . . .
8.7
Cerebral Amyloid Angiopathy . . . . . . . . . . . . . . . . . . . . . . . . . .
8.8
Soluble Ab as a Biomarker in the CSF . . . . . . . . . . . . . . . . . . . .

75
75
77
85
86
87
89
89
92

9

The Pattern of Lesions During the Transition to the Symptomatic
Phase and in Fully Developed Alzheimer’s Disease . . . . . . . . . . . . . . 95

9.1
NFT Stage III: Progression into the Basal Temporal Neocortex,
Including Portions of the Fusiform and Lingual Gyri,
Involvement of Superordinate Olfactory Centers and the Limbic
Thalamus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
9.2
Involvement of Neocortical Chandelier Cells . . . . . . . . . . . . . . . 99
9.3
Are Stages a–III Part of the AD-Associated Pathological
Process? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
9.4
Basic Organization of Insular, Subgenual, and Anterior Cingulate
Regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
9.5
NFT Stage IV: Further Progression of the Lesions into
Proneocortical and Neocortical Regions Governing High
Order Autonomic Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

7

The Pattern of Cortical Lesions in Preclinical Stages . . . . . . . . . . . .
7.1
Stages 1a and 1b: Development of Inclusions in Axons and of
Pretangle Material in Transentorhinal Pyramidal Cells . . . . . .
7.2
NFT Stages I and II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3
Prevalence of Stages a–II . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4
The Problem of Selective Vulnerability and the Potential

Transmission of Pathological Changes from One Neuron
to the Next . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.5
Imaging Techniques and Soluble Tau as Biomarker in
the CSF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


Contents

9.6
9.7

9.8

9.9

9.10

xi

Macroscopically Recognizable Characteristics of
Advanced AD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
NFT Stage V: Fan-Like Progression of the Neocortical Pathology
into Frontal, Superolateral, and Occipital Directions and its
Encroachment on Prefrontal and High Order Sensory
Association Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
NFT Stage VI: The Pathological Process Progresses Through
Premotor and First Order Sensory Association Areas into
the Primary Fields of the Neocortex . . . . . . . . . . . . . . . . . . . . .
The Pattern of the Cortical Tau Pathology in AD Mimics the

Developmental Sequence of Cortical Lipofuscin Deposits and,
in Reverse Order, That of Cortical Myelination . . . . . . . . . . . . .
The Prevalence of Tau Stages and Ab Phases in Various Age
Categories and Potential Functional Consequences of the
Lesions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

109

109

110

111

113

10

Final Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

131

11

Technical Addendum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.1 Stock Solution for Physical Developer . . . . . . . . . . . . . . . . . . .
11.2 Campbell-Switzer Technique for Brain-Amyloid Deposits . . . .
11.3 Gallyas Technique for Neurofibrillary Pathology . . . . . . . . . . .

.

.
.
.

135
137
137
138

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

141


Chapter 1

Prologue

Sporadic Alzheimer’s disease (AD) was first described by the physician Alois
Alzheimer (Fig. 2.1a) as an insidious and slowly progressive neurodegenerative
disorder of the human central nervous system (CNS) (Alzheimer 1906; Alzheimer
et al. 1995). Clinically, its earliest sign is a subtle decline in memory functions in a
state of clear consciousness. Intellectual and practical skills gradually worsen, and
personality changes manifest themselves, followed by deterioration of language
functions, impairment of visuospatial tasks, and, in the end, dysregulation of
autonomic functions and dysfunction of the motor system in the form of a
hypokinetic hypertonic syndrome (Albert et al. 2011; Morris et al. 2014). Both
the tempo of the cognitive decline and the duration of the disease as well as
neurological symptoms can vary considerably from one individual to another
(Franssen et al. 1993).

Clinically recognizable AD develops only in humans and is found in all ethnic
groups worldwide, whereby its prevalence increases remarkably with age (Purohit
et al. 2011). Of known diseases causing dementia, AD is the most frequent one and
accounts for 60–70 % of cases. In societies with increasing life expectancy, it can be
predicted that the number of demented individuals will quadruple by the year 2050,
whereby this estimate rests on the reasonable assumption that no significant therapeutic breakthrough for AD will occur in the immediate future (Brookmeyer
et al. 2007; Reitz et al. 2011; Mayeux and Stern 2012). As such, AD imposes an
enormous socio-political and economic threat that makes the search for causal or
symptomatic solutions an urgent medical priority (Mount and Downton 2006;
Trojanowski and Hampel 2011; Mayeux and Stern 2012).
At present, only a provisional clinical diagnosis can be made for AD, and this
usually takes place during the last phase of the underlying pathological process.
Diagnoses based on ante mortem observation, even with the help of advancing
ancillary disciplines, such as neuroimaging and biomarkers, are still too unreliable,

© Springer International Publishing Switzerland 2015
H. Braak, K. Del Tredici, Neuroanatomy and Pathology of Sporadic
Alzheimer’s Disease, Advances in Anatomy, Embryology and Cell Biology 215,
DOI 10.1007/978-3-319-12679-1_1

1


2

1 Prologue

thereby necessitating post mortem confirmation (Beach et al. 2012a; Montine
et al. 2012; Toledo et al. 2012; Serrano-Pozo et al. 2013).
Cases of sporadic AD show the presence of the same pathological process that

develops at specific CNS sites (Duyckaerts et al. 2009; Nelson et al. 2009, 2011).
Currently, cross-sectional studies performed on cohorts of non-selected autopsy
cases are the chief sources of information about the systematic progression of this
process (Bouras et al. 1994; Giannakopoulos et al. 1994; Braak and Braak 1997b;
Dickson 1997a; Duyckaerts and Hauw 1997; Braak et al. 2011). The present article
is based on the results of such analyses performed on brains conventionally fixed by
immersion in aqueous solutions of formaldehyde. The findings reported here
involve established histological methods (immunohistochemistry, silver stains,
dye-staining techniques) and can be reproduced by other laboratories.


Chapter 2

Introduction

2.1

Sporadic AD Is a Proteinopathy Linked
to the Development of Intraneuronal Inclusions
of Abnormal Tau Protein Which, in Later Phases, Are
Accompanied by the Formation of Extracellular
Plaque-Like Deposits of Amyloid-β Protein

Aggregates of abnormal proteins are the hallmarks of the AD-associated pathological process. Intraneuronal inclusions consisting of aggregated protein tau develop
only in vulnerable types of CNS nerve cells (Fig. 2.1b) (Goedert and Spillantini
2006; Goedert et al. 2006; Mandelkow et al. 2007; Alonso et al. 2008; Iqbal and
Grundke-Iqbal 2008; Iqbal et al. 2009; Kolarova et al. 2012; Mandelkow and
Mandelkow 2012; Spillantini and Goedert 2013) whereas plaque-like deposits
containing amyloid-β protein (Aβ) appear in the extracellular space of the CNS
(Fig. 2.1c) (Beyreuther and Masters 1991; Selkoe 1994, 2000; Mattson 2004:

Masters and Beyreuther 2006; Haass and Selkoe 2007; Alafuzoff et al. 2009;
Haass et al. 2012; Masters and Selkoe 2012; Selkoe et al. 2012). Both types of
proteinaceous deposits appear at different times within different CNS predilection
sites and, from there, spread out systematically into previously uninvolved brain
regions. In the course of the pathological process, both types of lesions increase in
severity. The AD process begins with intraneuronal aggregations of the protein tau
followed, after a time-lag of approximately a decade, by the extracellular deposition
of Aβ (Braak and Braak 1997b; Duyckaerts and Hauw 1997; Silverman et al.
1997; Braak et al. 2013) (Fig. 9.16, Tables 9.2 and 9.6). Neuritic plaques (NPs),
i.e., combined deposits consisting of insoluble Aβ with dystrophic neurites that
contain aggregated tau, develop only in the late phase of the disease process (Braak
and Braak 1997b; Braak et al. 2013).

© Springer International Publishing Switzerland 2015
H. Braak, K. Del Tredici, Neuroanatomy and Pathology of Sporadic
Alzheimer’s Disease, Advances in Anatomy, Embryology and Cell Biology 215,
DOI 10.1007/978-3-319-12679-1_2

3


4

2 Introduction

Fig. 2.1 (a) Alois Alzheimer. (b) Neurofibrillary lesions shown by Gallyas silver-iodide staining
technique. (c) Aβ plaques visualized by Campbell-Switzer silver-pyridine staining technique. (d)
The lesions (red) increase in severity and extent without remission during a long presymptomatic
disease phase that reaches a shorter and final symptomatic phase. (e–g) Selective neuronal
vulnerability in sporadic AD. Neuronal types protected against AD-associated pathology include

2.1 Sporadic AD Is a Proteinopathy Linked to the Development of Intraneuronal. . .

5

In contradistinction to the pathology that emerges in the course of sporadic
Parkinson’s disease (PD) (Braak and Del Tredici 2009; Del Tredici et al. 2010; Del
Tredici and Braak 2012), the hallmark lesions associated with AD remain almost
exclusively confined to the CNS. Nerve cells of the peripheral and enteric nervous
systems (PNS, ENS) likewise contain normal tau and the amyloid precursor protein
(APP), but only the olfactory epithelium is known to develop abnormal protein
aggregates (Arnold et al. 2010; Kova´cs 2013). Why is it that the ENS and other PNS
sites do not develop aggregated tau and Aβ? Attempts to understand the pathogenesis of the hallmark lesions should also explain why the AD process develops
almost exclusively within the CNS.
The AD-associated pathological process, once started, is not known to regress,
improve spontaneously, or go into remission. This controversial but important
aspect of the AD process is treated below in Sect. 2.3. The process spans nearly a
lifetime unless it is interrupted by death from other causes. In other words, it
encompasses a much longer time-span than the clinically recognizable phase and
consists of a relatively short symptomatic final phase and a long preclinical phase.
The last phase, accompanied by the loss of cognitive and executive functions,
manifests itself, as a rule, only at an advanced age (Fig. 2.1d).
The hallmark lesions are generally accompanied in this late phase by pathologies
attributable to disorders that develop with increasing age (e.g., vascular disease,
metabolic syndrome, concomitant neurodegenerative diseases) that aggravate, to
varying degrees, the clinical picture, making the diagnosis and the questions
surrounding the pathogenesis of AD increasingly complex (van Gool and
Eikelenboom 2000; Iadecola 2004, 2010; Fotuhi et al. 2009; Duyckaerts
et al. 2009; Grinberg and Thal 2010; Chui et al. 2012; Hunter et al. 2012; Korczyn

et al. 2012; Korczyn 2013; Thal et al. 2012; Wang et al. 2012; Kova´cs et al. 2013;
Serrano-Pozo et al. 2013). The co-occurrence of sporadic AD and the lesions
associated with sporadic PD or frontotemporal lobe degeneration (FTLD) is especially problematic (Duyckaerts et al. 2009; Mesulam et al. 2014). Nevertheless,
owing to the co-occurrence of multiple pathologies, each case is unique. For this
reason, sporadic AD is not always viewed as a single disease entity but as a
syndrome leading to dementia (Korczyn 2013).
In youth and in early adulthood Aβ plaque deposition is non-existent or rare. Tau
aggregates, on the other hand, occur before puberty and are absent only in very
young children (Fig. 9.12) (Braak et al. 2011; Braak and Del Tredici 2011, 2012).
The ongoing development of both the intraneuronal tau aggregates and the extracellular plaque-like Aβ deposits is extraordinarily slow, so that the hallmark lesions
cannot be said to originate only in old age or typically during aging (Morrison and
Hof 1997; Nelson et al. 2011). Nonetheless, clinically overt AD has been viewed as
ä
⁄
Fig. 2.1 (continued) short-axoned projection cells and local circuit neurons (e) and pyramidal cell
with a long and heavily myelinated axon (f). Vulnerable types of pyramidal cells have a long and
sparsely myelinated axon (g). (e–g adapted with permission from H Braak and K Del Tredici, Adv
Anat Embryol Cell Biol 2009;201:1–119)


6

2 Introduction

a disorder that is intrinsic to the aging process or, at the very least, indirectly
attributable to it. Age-related factors capable of damaging aging postmitotic cells,
such as chronic inflammation, oxidative stress, mitochondrial and metabolic dysfunction, blood-brain barrier impairment, or failure of the ubiquitin-proteasomal
system, are viewed as pivotal factors in the pathogenesis of AD (de la Monte and
Tong 2013; Pohanka 2013; Yan et al. 2013; Arshavsky 2014). None of these factors
alone, however, suffices to explain why the hallmark lesions consistently and

selectively develop in only a few types of nerve cells. Obviously, they fail to affect
all known types of postmitotic cells inside and outside of the CNS. Even when the
discussion is confined to the CNS, it is evident that AD does not involve all
neuronal types there indiscriminately. Rather, the AD-process is a remarkably
selective one in that it develops in only a minority of neuronal types while sparing
all of the rest. In addition, the effects of age-related factors do not explain why the
tau lesions develop in children and young adults during the early phase of the
disease. Thus, advanced age per se is not necessary for the formation of the
AD-related tau lesions: The pathological process underlying sporadic AD is not
‘age-dependent’ but an uncommonly protracted and progressive process that frequently extends into old age (Fig. 2.1d). Clinical symptoms develop subtly, and
nerve cell impairment leads to a gradual loss of fundamental functions that first
appear after a given threshold is exceeded (Fig. 2.1d).
The present monograph rests upon the assumption that it is the pathological
process depicted above which, in its final phase, causes the clinical symptoms of
AD. Individuals with a history of cognitive dysfunction, whose brains do not have
the hallmark AD lesions, should be classified in the heterogeneous group of
non-AD dementias (Tolnay and Probst 1999; Clavaguera et al. 2013a; Dickson
et al. 2007).

2.2

Some Neuronal Types Exhibit a Particular Inclination
to the Pathological Process While Others Show
a Considerable Resistance To It

The deterioration of the CNS in sporadic AD specifically targets predilection sites
in select subcortical nuclei and cortical areas. Of the numerous neuronal types
within the CNS, only some develop AD-related pathology, whereas others, including those directly in the vicinity of involved nerve cells, remain morphologically
and functionally intact (Braak and Braak 1999). The resultant non-random regional
distribution pattern of the lesions is reflected by dysfunction of select neuronal

types and, in some regions, mild neuronal loss. The pathological process selectively
affects high-order processing regions (Arendt 2000). However, because these
regions are not absolutely essential for survival the disease process can exist and
progress for nearly an entire lifetime until death, which is usually caused by severe
autonomic dysregulation.


2.2 Some Neuronal Types Exhibit a Particular Inclination to the Pathological. . .

7

Most of the vulnerable neuronal types are phylogenetically late-appearing elements that also achieve functional maturity late in life (Rapoport 1988, 1989;
Bartzokis 2004; Rapoport and Nelson 2011). These nerve cells frequently retain a
high degree of structural plasticity in the adult brain and show signs of immaturity
that endure well into adulthood (Stephan 1983; Rapoport 1990, 1999; Arendt
et al. 1998; Arendt 2000; Bufill et al. 2013). The distalmost segments of their
dendrites often display a slow and ongoing growth pattern that persists even when
dendritic maturation in non-susceptible nerve cell types is already complete (Arendt
2000). In addition, the vulnerable cells frequently display a loss of regulation over
neuronal differentiation with a partial reactivation of the cell cycle (Arendt 2004,
2005, 2012).
Nearly all of the diverse types of nerve cells in the CNS that are prone to develop
AD-related tau aggregates are projection neurons, i.e., nerve cells with a localized
dendritic arbor and an axon that is disproportionately long in relation to the size of
the host cell soma (Fig. 2.1f, g). Inasmuch as glutamatergic, gabaergic, dopaminergic, noradrenergic, serotonergic, histaminergic, and cholinergic projection cells
become involved, the type of neurotransmitter or neuromodulator synthesized is not
essential for predicting which nerve cells are especially vulnerable or predisposed
to the AD process. Short-axoned neurons generally do not become affected
(Fig. 2.1e) (Braak and Braak 1999; Benarroch 2013) with the rare exception of
large cholinergic local circuit neurons in the striatum and chandelier cells in the

basal temporal neocortex (see Sect. 9.2). In addition, nearly all projection neurons
with a short axon remain intact, such as the small pyramidal cells in neocortical
layers II and IV (spiny stellate cells) (Fig. 2.1e) (DeFelipe et al. 2002) and those of
the presubicular parvocellular layer. An aggrecan-based perineuronal net surrounds
subsets of cortical and subcortical projection neurons, and these subsets do not
develop intraneuronal tau lesions. As a result, perineuronal nets may contribute to
the selective resistance of these nerve cells against the tau-mediated pathological
process (Morawski et al. 2010). The overall resistance on the part of short-axoned
neurons has its consequences: When a projection essentially synapses only on local
circuit neurons—e.g., as is the case with neocortical pyramidal projections from
layer V to layer III and those from layer VI to layer IV (Bannister 2005)—the
pathological process cannot spread any further. The potential route of propagation
from projections of the anterior subnuclei of thalamus to the presubicular
parvocellular layer also comes to a halt at the short-axoned neurons there.
All of the endangered neuronal types generate a long and thin-caliber axon that
either is encased by a thin myelin sheath or does not undergo myelination
(Fig. 2.1g). Projection neurons reach full functional maturity only after their
axons have achieved their stipulated degree of myelination (van der Knaap
et al. 1991). Human neocortical projection neurons in prefrontal or in high-order
sensory association areas commence myelination late in life and, thus, are thinly
myelinated and predisposed to the AD-related process (Bartzokis 2004). By contrast, cortical or subcortical projection neurons with heavily myelinated axons resist
developing tau aggregates (Fig. 2.1f). These include the Betz cells in the primary


8

2 Introduction

motor area, Meynert’s pyramidal cells in the striate area, or host neurons of the
medial longitudinal fascicle.

Increased thickness of the myelin sheath provides greater velocity of axonal
conduction and, at the same time, a considerable reduction of the metabolic
demands placed on the host cell for the transmission of the impulses (Fig. 2.1f).
By contrast, rapid-firing projection neurons with an unmyelinated or immaturely
myelinated axon are subject to higher energy turnovers and are thereby chronically
exposed to oxidative stress (Fig. 2.1g) (Pohanka 2013; Yan et al. 2013). The
relatively postponed onset of myelogenesis in neurons of high-order regions
which are not absolutely essential for preservation of basic brain functions results
in high energy consumption (Fig. 2.1g). Yet, it is precisely these neurons that enrich
and optimize complex activities, such as learning, memory, and perception, that are
particularly prone to develop AD lesions (Arendt 2000). These include latematuring pyramidal cells, whose axons preferentially develop connections to the
distalmost dendritic segments of other cortical pyramidal cells but have no immediately obvious functions.
The myelin sheath provides a mechanical barrier against viruses and other
pathogens by virtually isolating the axon from the surrounding extraneuronal
space. However, this also means that the energy supply for long axons cannot
originate in the cell soma. Instead, glial cells increasingly assume this function. The
axon is embedded among oligodendroglial cells that protect and support it. Local
astrocytes take up substances critical for the energy balance via contacts to the
cerebral vasculature and redistribute them to oligodendrocytes by means of gap
junctions. Additional mechanisms enable the transfer of these substances from the
oligodendrocyte to the axon (Nave 2010; Lee et al. 2012b).
Nerve cells of the human adult generally are richly supplied with lipofuscin or
neuromelanin granules (Braak 1980; Double et al. 2008) and, notably, all of the
vulnerable cell types contain such granules (Fig. 2.1g). The presence of lipofuscin
or neuromelanin deposits alone, however, does not suffice to account for the
susceptibility of projection cells to the AD-process because many of them develop
large amounts of these paraplasmatic granules with advancing age without developing tau aggregates, as, for instance, the Betz cells of the motor cortex or
projection neurons of the lateral geniculate body. On the other hand, nerve cells
that conspicuously lack lipofuscin or neuromelanin granules, or that contain only a
few granules even in old age, consistently resist the pathological process. Prime

examples are the large projection neurons of the hypothalamic lateral mamillary
nucleus where, even in old age, lipofuscin granules are hardly present and
AD-associated tau aggregates do not develop.
Viewed against this background, the combination of paraplasmatic pigment
granules and an unmyelinated or sparsely myelinated long and thin-caliber axon
in phylo- and ontogenetically late-maturing projection neurons appears to be a
deficiency of the human CNS that may be necessary for the induction of the
AD-associated process (Fig. 2.1g) (Braak and Braak 1999).


2.3 Consistent Changes in the Regional Distribution Pattern of Intraneuronal. . .

2.3

9

Consistent Changes in the Regional Distribution
Pattern of Intraneuronal Inclusions Make a Staging
Procedure Possible

As in other illnesses, at some point during the pathological process, patients cross a
threshold from the preclinical phase to the symptomatic manifestation of AD
(Fig. 2.1d). By the time clinicians make their diagnosis, patients are, relatively
speaking, in the late phase of a larger pathological process. The disease festers in
the CNS for decades until its dimensions are such that dysfunctional behavior
becomes manifest (Dubois et al. 2010).
Cases with clinically recognizable symptoms usually can be assigned to one of
four neuropathological subgroups (neurofibrillary stages III, IV, V, VI), which
differ with respect to the topographic extent of the AD-related tau pathology
(Fig. 2.2a, e). The idea of regional expansion rests on the assumption that the

lesions most likely develop in a consecutive manner within the CNS and then
increase in severity and extent (Fig. 2.2b). Each subgroup, therefore, displays newly
affected regions in addition to the tau lesions existing at previously involved sites
(Braak et al. 2011).
Tau aggregates occur as incidental findings in non-symptomatic individuals
(Linn et al. 1995; Dubois et al. 2010; Ferrer 2012), and these also can be divided
into one of four subgroups (Fig. 2.2c, d). These lesions sometimes are viewed as a
variant of neuronal aging, neuroprotective, or as possible markers of a non-ADrelated tauopathy (Attems et al. 2012; Cower and Mudher 2013; Jack and Holtzman
2013; Jack et al. 2013; Thal et al. 2013; Braak and Del Tredici 2014; Crary et al.
2014; Kuchibhotla et al. 2014). However, they also can be interpreted as markers of
early phase disease—comparable to malignant cells in a carcinoma that fail to
produce symptoms but mark the onset of a pathological process (Fig. 2.2e). The
concept that incidental tau aggregates are completely ‘normal’ requires the highly
problematic definition of the point at which such tau inclusions convert from a
‘normal’ status into ‘disease-related’ lesions. Disease-related lesions existing prior
to the clinical manifestation of a disease are usually regarded as prodromes. As a
clinical entity, AD includes dementia, but the AD-related pathological process
includes a very protracted preclinical phase, which certainly occupies a pivotal
position in relation to the pathogenesis of AD (Figs. 2.1d and 2.2e).
We view such clinically mute incidental tau lesions as a potential threat to the
CNS for the following reasons: The presymptomatic and symptomatic disease
phases are both marked by the presence of the same types of intraneuronal tau
aggregates in the same types of nerve cells and at the same regional predilection
sites. Second, since the lesional pattern of the last preclinical subgroup closely
resembles that of the first symptomatic subgroup (compare Fig. 2.2d and b), both
sets of subgroups combined can be taken to reflect the full spectrum of the
pathological AD-process (Fig. 2.3a–c). The lesions develop in a remarkably predictable and consistent sequence across cases (Figs. 2.2e, 9.8, and 9.13) (Kemper
1984; Arnold et al. 1991; Braak and Braak 1991a, 1997a, b, 1999; Braak

10

2 Introduction

Fig. 2.2 Presymptomatic and symptomatic phases of sporadic AD. (a) Most symptomatic cases
with AD-associated tau pathology fall into four subgroups. (b) Given the consistency of this
finding and, based on the topographic distribution pattern of the lesions, the four groups can be


2.3 Consistent Changes in the Regional Distribution Pattern of Intraneuronal. . .

11

et al. 2006a; Hyman and Gomez-Isla 1994; Duyckaerts and Hauw 1997; Delacourte
et al. 1999). Third, the existence of asymmetric tau distribution patterns, as seen in
double hemispheres sections immunostained for AT8, indicates that a neurobiological continuum of tau pathology exists and that this pathology also tends to progress
within one and the same individual, albeit at a different pace (Fig. 2.4a–c).
Locating the first tau aggregates in an organ as voluminous as the human CNS
might seem like an insurmountable undertaking, but it is possible provided the
predilection sites of the pathological process are known. The lesions do not develop
randomly, here at one site, there at another. Instead, the AD process is stereotypic,
beginning in the same regions and advancing with little inter-individual variation.
As a rule, one does not see appreciable differences bilaterally with respect to the
topographic distribution of tau pathology in double hemisphere sections (Fig. 2.3),
and when discrepancies occur, they do not amount to more than one (Figs. 2.3a and
2.4a) or two stages (Fig. 2.4b, c). In cases with advanced stages, this phenomenon
disappears and the hemispheres of such cases display a more or less symmetric
involvement (Fig. 2.3c). In this context, it should be noted that the concept of
neuropathological staging is recommended for practical reasons only. In principal,
it is an artificial construct because, as pointed out above, the hallmark lesions

develop continually rather than in definite steps (Braak and Braak 1991a).
The AD-associated process begins with the appearance of non-argyrophilic tau
lesions in stages a-c in brainstem nuclei that diffusely project to the cerebral cortex
and progresses from there into the cerebral cortex, i.e., the transentorhinal region
(non-argyrophilic cortical lesions in stages 1a and 1b) (Fig. 5.3). Thereafter, the
inclusions partially convert into argyrophilic lesions that characterize NFT stages
I–VI. From the transentorhinal region (stage I), the pathology advances into the
entorhinal region and hippocampal formation (stage II) (Fig. 7.3). During stage III,
tau lesions encroach upon the adjoining basal temporal neocortex (Fig. 9.2). In
stage IV, they extend more widely into the temporal neocortex, insula, subgenual
and anterogenual frontal regions, and anterior cingulate areas (Fig. 9.8). Stage V
cases display severe involvement of most neocortical association areas, leaving
only first-order association areas and primary fields mildly involved or intact.
Finally, nearly all cortical areas show neurofibrillary changes in stage VI (Table 2.1;
Figs. 2.2e, 2.3c, and 9.13) (Braak and Braak 1991a; Braak et al. 2006a). Abnormal
tau formation continues to take place from the beginning (stage a) until the end

ä
⁄
Fig. 2.2 (continued) arranged to show disease progression (neurofibrillary stages III, IV, V, VI).
(c) Similarly, most non-symptomatic cases also fall into four subgroups (d) and can be ordered
sequentially (pretangle stages a–c, 1a/1b, and NFT stages I, II). (e) Mild to moderate tau lesions
develop over time until a threshold from the prodromal to the symptomatic (clinical) phase is
crossed. Roman numerals correspond to stages of Gallyas-positive (argyrophilic) lesions. Arabic
numerals represent stages with cortical AT8-ir (non-argyrophilic) lesions (stages 1a/1b), whereas
lower case letters designate stages with subcortical AT8-ir non-argyrophilic lesions (stages a–c).
See also Table 2.1


12

Fig. 2.3 Overview of
AD-related
AT8-immunopositive tau
stages in three double
hemispheres of 100 μm
thickness. (a) The
hemispheres of this
cognitively intact 80-yearold female display NFT
stages I (left) and II (right)
in the absence of Aβ
plaques. (b) NFT stage III in
a 90-year-old female, who
died of a malignant
pancreatic neoplasm. Aβ
plaques were also present.
(c) Frontal section from a
severely demented 72-yearold female AD patient
(cause of death aspiration
pneumonia) with bilateral
NFT pathology and
ventricular widening typical
of stage VI. Aβ plaques
were also present (adapted,
in part, with permission
from H Braak and K Del
Tredici, Alzheimers
Dement. 2012; 8:227–233).
Scale bar in (c) is valid for
(a) and (b)

2 Introduction


2.3 Consistent Changes in the Regional Distribution Pattern of Intraneuronal. . .
Fig. 2.4 Double
hemispheres of 100 μm
thickness showing
asymmetrical AD-related
AT8-immunopositive tau
stages. (a) Section from a
cognitively intact 77-yearold female (cause of death
myocardial infarction) at
NFT stage III (left) and
stage II (right). No
ventricular widening is
detectable. (b) Frontal
section from a 75-year-old
female patient (cause of
death chronic lymphatic
leukemia) with NFT stages
III (left) and NFT stage I
(right). (c) This frontal
section from a cognitively
impaired 75-year-old male
(cause of death metastatic
pulmonary neoplasm)
displays pathology
corresponding to NFT
stages II (left) and IV
(right). Deviations of more

than one NFT stage are
unusual (compare Fig. 2.3).
See also Sect. 9.3. Scale bar
in (c) applies also to (a) and
(b)

13